Pocket-sized counter tube instruments for detecting radioactivity are known. However, the miniature Geiger counters presently available are not very sensitive and are therefore not suited to the detection of slightly radioactive contamination in the environment. Specialized contamination monitors are generally used for that purpose, being equipped with special, highly sensitive counter tubes. The construction of such instruments, however, is so expensive that there could be no question of their use except by professionals because of the high cost involved. The basic situation thus described above in general will now be discussed in greater detail.
Some of the requirements to be made of a reliable radiation measuring instrument are set forth in the DIN 6818 and DIN 44 801 standards. It must furthermore be noted that a hand instrument for measuring contamination in the lowdose range must have a certain minimum sensitivity, so that a brief and/or slight increase in the counting rate above the average background level occurring in the course of a scanning movement can also in fact be recognized by the user of the device.
In practice, the only suitable contamination measuring devices have been those which respond to the natural background level of approximately 10 microrem per hour with a counting rate of 1 s.sup.-1 or more. At low counting rates, it is extremely difficult to detect variations in the counting rate directly, regardless of the manner in which the counting rate is registered or displayed. When this is done using an indicator instrument, then even when the maximum permissible time constant of 4 s (DIN 44 801) is used, the reading accuracy at low counting rates is insufficient. Optical or acoustical single-pulse registration produces no better results, because the human organism is not capable of perceiving with sufficient accuracy stochastic events which elapse slowly.
From the above discussion it will be understood that the low-voltage counter tubes having a noble-gas/halogen atmosphere presently available and of standard design, and in particular the miniature versions, cannot be considered appropriate for the construction of an efficient radiation measuring device suitable for measuring contamination, because the sensitivity of such standard detectors is insufficient for reliably detecting weak radiation.
In recognition of this fact, the use of proportional counter tubes of large surface area was advocated at quite an early date. Proportional counter tubes are known which have been optimized, at relatively great expense, in the direction of maximum possible sensitivity and which are designed as measuring instruments for alpha and beta radiation, being particularly in the form of liquid flow counters having thin-walled windows. Window-type counter tubes of this kind must, because of their construction, all be operated with a constant high voltage of several kilovolts, and therein lies one of the primary difficulties in the miniaturization of highly sensitive radiation measuring instruments.
Portable embodiments having a built-in, large-surface proportional counter tube are also known, which are commercially available as workplace monitors in order to satisfy the requirements of Article 64 of the Radiation Protection Regulations of the Federal Republic of Germany. Given the considerable expense of these instruments, their structural volume of more than 2 dm.sup.3 and a weight of more than 1 kg, such devices are used practically only in connection with the handling of radioactive substances requiring official permission according to Article 3 of the Radiation Protection Regulations of the Federal Republic of Germany. Universally useable pocket instruments having large-surface proportional counter tubes have not heretofore been known. The only pocket instruments known for this purpose have been those with Geiger-Miller counter tubes, particularly counter tubes of the low-voltage type and of classic design.